In recent years, there has been a great deal of industrial emphasis on conducting methanol carbonylation reactions under conditions wherein the water content in the reaction is less than about 14%.
The ability to perform methanol carbonylation reactions at these water levels results in significant economic benefits since equipment costs and energy requirements are reduced. Furthermore, rhodium (Rh) losses are reduced in low water level systems; rhodium typically being a component of traditional carbonylation catalyst systems. And, since rhodium is very expensive, even small reductions in catalyst loss can result in substantial savings. The problem, however, in many prior art carbonylation processes is that as the amount of water in the reaction is reduced, the concentration of the active catalyst species is lowered, the net effect being that the reaction rate decreases.
In order to overcome the aforementioned problems typically associated with low water carbonylation processes, various additives have been introduced into prior art carbonylation catalyst systems the goal being to increase the reaction rate of the carbonylation process at low water levels. In particular, it has been a practice of the prior art to incorporate an alkali metal halide, such as lithium iodide (LiI), in the carbonylation reaction medium to improve the carbonylation reaction rate and product yield. The following references enunciate the current state of the art in methanol carbonylation wherein LiI is added to increase the reaction rate and yield of the process:
U.S. Pat. Nos. 5,214,203 and 5,391,821 both to Koyama et al. provide processes for producing a carboxylic acid, such as acetic acid, by reacting an alcohol or its derivative with carbon monoxide in the presence of a catalyst system which contains a rhodium component, an alkyl halide, water and an iodide salt, such as lithium iodide. The references disclose that when an extremely large amount of an iodide salt (at least 0.3 mol/liter) is incorporated in the reaction solution, the formation of unwanted side products such as methane, can reportedly be controlled with concurrent improvement in the carbonylation rate.
U.S. Pat. No. 5,003,104 to Paulik et al. provides another carbonylation process which adds LiI to the carbonylation reaction in order to improve the reaction rate of the process. Specifically, the Paulik et al. reference is directed to a process for the carbonylation of a carbonylatable reactant, such as alkyl ester, dialkyl ether, alkyl alcohol or olefin, by reacting same with carbon monoxide. More specifically, the reference discloses a carbonylation process wherein the reaction is conducted in the presence of a catalyst system which comprises a rhodium compound and a halogen-containing promoter, at a temperature from about 50.degree. C. to about 400.degree. C. and a CO partial pressure of 1 to about 15,000 psi. A mixture of LiI and CH.sub.3 I are among the various halogen-containing promoters disclosed in the reference.
U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 to Smith et al. relate to processes for the production of acetic acid which comprise reacting methanol with carbon monoxide in a liquid reaction medium containing a rhodium catalyst, water, acetic acid, methyl acetate, lithium iodide and methyl iodide. The object of the Smith et al. references reportedly lies in catalyst stability and reactor productivity as manifested by maintaining in the reaction medium, along with a catalytic-effective amount of rhodium, a finite concentration of water (at least 0.1 weight percent) and methyl acetate and methyl iodide in specified portions.
U.S. Pat. No. 5,281,751 to Schreck provides a process for preparing aliphatic carboxylic acids of the formula RCOOH, wherein R is an alkyl group having 1 to 5 carbon atoms, comprising the catalytic reaction of an alcohol of the formula ROH and carbon monoxide in the presence of a rhodium catalyst, methyl iodide, lithium iodide (at high content), water (at low content, i.e. 0 to 6.5% by weight) and an organic ester of the formula RCO.sub.2 R, R being defined as above. The process can optionally be carried out in the presence of hydrogen and/or an organic ligand of the formula ER".sub.3 wherein E is nitrogen, phosphorous, arsenic, antimony or bismuth and R" is an organic moiety. The concentration of the organic ligand employed in the reference is from about 50:1 to about 10:1.
U.S. Pat. No. 5,416,237 to Aubigne et al. relates to an improved process for producing acetic acid by carbonylating methanol in the presence of carbon monoxide, a rhodium carbonylation catalyst, methyl iodide, a carbonylation catalyst stabilizer such as LiI, water, methyl iodide, methyl acetate and acetic acid. Specifically, this reference maintains a finite concentration of water, up to about 10% by weight, and a methyl acetate concentration of at least 2% by weight in the liquid reaction composition and recovers acetic acid by passing the liquid reaction composition through a flash zone to produce a vapor fraction which is then passed to a single distillation column. By maintaining the above concentration of water and methyl acetate in the liquid reaction composition, Aubigne et al. reportedly obtains highly pure acetic acid having a water content of less than 1500 ppm and a propionic acid concentration of less than 500 ppm.
In each of the aforementioned references, the water content of the carbonylation reaction is reduced and the reaction rate is maintained by the addition of an alkali metal halide, e.g. LiI, to the reaction. It is, however, suspected that alkali metal halides, such as LiI, promote stress crack corrosion of the reactor vessel. Thus, it would be of great benefit if a process could be developed that reduces the water content in the carbonylation reaction while maintaining catalyst stability and high reaction rates without the need of adding LiI or any other alkali metal halide to the carbonylation reaction system.